I have had success using this more than once. All of the necessary config settings are in the "Settings" section of your gmail account. Under the assumption that you already know that, the following illustrates what I use:

In the Send Email script step choose to send as smtp.

In the window that comes up I use the following configuration:

User:

Name: "username" (I am assuming you can assign a more user-friendly name here as a "From" label)

Email Address: "username@gmail.com"

Reply To Address: "reply.to.username@gmail.com" (this can be the same as the email address above, as I do. I have not tested it with a different address)

Server:

Outgoing SMTP Server: "smtp.gmail.com"

Port Number: 465 (data type is as a number but I suspect with FileMaker this does not much matter. Alternatively port 587 may also be used but I recall having some issues)

Use SSL: Yes (checkmark the box)

Authentication: Plain Password

User Name: "username" (does not include "@gmail.com")

Password: "password"

Also, be sure that you have enabled POP access in your gmail account under "Settings" > "Fowarding and POP/IMAP".

If this all reflects what you currently are using or does not appear to work then try posting your settings so that we may have a look.